Satellite communication systems have emerged as a viable solution for supporting broadband services. As such, power and bandwidth efficient modulation and coding are highly desirable for satellite communications systems to provide reliable communication across noisy communication channels. These communication channels exhibit a fixed capacity that can be expressed in terms of bits per second (bps) per Hz (bps/Hz) for a given signal-to-noise ratio, defining a theoretical upper limit (known as the Shannon limit). As a result, coding design has aimed to achieve rates approaching this Shannon limit. One such class of codes that approach the Shannon limit is Low Density Parity Check (LDPC) codes.
Traditionally, LDPC codes have not been widely deployed because of a number of drawbacks. One drawback is that the LDPC encoding technique is highly complex. Encoding an LDPC code using its generator matrix would require storing a very large, non-sparse matrix. Additionally, LDPC codes require large blocks to be effective; consequently, even though parity check matrices of LDPC codes are sparse, storing these matrices is problematic. From an implementation perspective, storage is an important reason why LDPC codes have not become widespread in practice. A key challenge in LDPC code implementation has been how to achieve the connection network between several processing engines (nodes) in the decoder.
The explosive growth of broadband services has been fueled by consumers' demands for greater and greater data rates to support, for example, their multi-media applications (e.g., streaming video, web surfing, etc.). Therefore, communication service providers require an infrastructure that can support high data rates, particularly in bandwidth-constrained systems. Higher-order modulation techniques that carry more than two bits per symbol, such as 8-PSK (Phase Shift Keying) and 16-QAM (Quadrature Amplitude Modulation), can provide more efficient bandwidth utilization. Unfortunately, conventional LDPC communication systems utilize a modulation scheme that carries binary or quaternary modulation whose signal waveforms representing the information are either binary anti-podal, or orthogonal to each other.
Therefore, there is a need for a satellite communication system that employs simple encoding and decoding processes, while minimizing cost. There is also a need for using LDPC codes efficiently to support high data rates, without introducing greater complexity. There is also a need to improve power and bandwidth efficiencies in a bandwidth constrained system, such as a satellite communication system.